Hypercholesterolemia, especially high serum level of low-density lipoprotein (LDL) cholesterol, has been revealed to be a risk factor of arteriosclerotic diseases by a number of epidemiological surveys. Actually, drugs capable of decreasing LDL cholesterol level such as 3-hydroxy-3-methylglutaryl-coenzyme A (HMG-CoA) reductase inhibitors have been used with the aim of preventing coronary artery diseases, and demonstrated to have some benefits in many large scale clinical tests. However, their preventive effect on coronary diseases is limited to some extent, and is not satisfactory enough yet.
Recently, low serum level of high density lipoprotein (HDL) cholesterol has been revealed to be a potent risk factor of arteriosclerotic diseases by a number of epidemiological surveys and large scale clinical tests. HDL is known to have various antiarteriosclerotic effects and attention is focused on the potentiality of drugs increasing HDL cholesterol level as a means for prevention or treatment of arteriosclerotic diseases. However, there are no drugs that can be used in a satisfactory manner for this purpose. Fibrates and HMG-CoA reductase inhibitors have only low activity of increasing HDL cholesterol level; nicotinic acid derivatives can significantly increase HDL cholesterol level, but have serious toleration issues. Accordingly, there has been a demand for a well-tolerated agent which can significantly elevate HDL cholesterol level, thereby preventing or reversing the progression of atherosclerosis.
It is known that many proteins are involved in the regulation mechanism for catabolism of various lipoproteins. Among them, the role of cholesteryl ester transfer protein (CETP) became to draw attention. CETP is a protein responsible for transfer of cholesteryl ester (CE) and triglyceride between lipoproteins, and mediate the transfer of CE from HDL to LDL or to very low density lipoprotein (VLDL). Accordingly, CETP activity affects greatly the lipid composition in lipoprotein particles. For example, it is known that administration of an active neutralizing monoclonal antibody against CETP to rabbit or hamster elevates HDL cholesterol level and lower LDL cholesterol level. Furthermore, human being having decreased or eliminated CETP activity due to gene mutation shows raised blood HDL cholesterol level and lowered blood LDL cholesterol level. On the other hand, it is known that transgenic mice and rats made to express CETP show lowered HDL cholesterol level and raised LDL cholesterol level. Thus, it is considered that CETP greatly contribute to the regulation of serum lipids, and thereby affecting the change of serum lipid profile such as decrease of HDL cholesterol level and increase of LDL cholesterol level. Accordingly, it is assumed that a high value of CETP activity would induce arteriosclerosis.
In fact, CETP activity varies depending on animal species. It is known that, arteriosclerotic lesions are readily formed by cholesterol loading in animals with high CETP activity such as rabbits, whereas such lesions hardly occur in animals with low CETP activity such as rats. Furthermore, it is confirmed that continuous suppression of CETP activity by administration of antisense oligodeoxynucleotide resulted in effects such as increase of blood HDL cholesterol level and reduction in arteriosclerotic lesions in cholesterol-fed rabbits.
The above findings indicate that CETP activity is in negative correlation with HDL cholesterol, and that inhibition of CETP activity would decrease the degree of risk for arteriosclerotic diseases. It is therefore expected that compounds capable of inhibiting CETP activity can block the transfer of cholesterol from HDL to LDL or VLDL, and thereby increasing HDL cholesterol that tends to prevent arteriosclerosis while lowering LDL cholesterol that tends to promote arteriosclerosis. In this way, such compounds can serve as a useful preventive or therapeutic agent for arteriosclerotic diseases, hyperlipemia or dyslipidemia and provide effective medical treatment for the first time.
Examples of compounds having CETP inhibitory activity include tetrahydroquinoline derivatives. See, PCT International Publication WO00/17164 pamphlet, PCT International Publication WO00/17165 pamphlet and PCT International Publication WO00/17166 pamphlet.
However, these compounds have defects. That is, they are poorly soluble in water and cannot be absorbed enough in vivo, a sufficient blood level for taking medicinal effect can hardly be achieved even when administered as an ordinary formulation for oral administration. See, WO03/63868.
Accordingly, it has been demanded to find a novel compound which eliminates the above-mentioned defects and intensive studies have been done on dibenzylamine type compounds, and the like. See, PCT International Publication WO05/100298 pamphlet, PCT International Publication WO04/020393 pamphlet, PCT International Publication WO 06/056854 pamphlet and JP 2003-221376 A.